As shown in FIG. 1, a solidly mounted or acoustic mirror type thin film bulk acoustic wave resonator filter 10 of the state of the art includes a resonator section 11 based on a piezolayer 19 of piezoelectric material, such as ZnO or AlN, and includes an acoustic mirror 12, all mounted on a substrate 14 made for example, from glass. The thin film bulk acoustic wave resonators 16, 18 convert sound waves to electric signals, and vice versa, and can be used as a filter in electronic circuits because of its frequency dependent electrical impedance.
Typically, the acoustic mirror 12 of the thin film bulk acoustic wave resonator filter 10 is formed from a combination of reflecting layers 24 to 30 of materials of differing acoustic impedance. An acoustic mirror 12 is built up on the substrate 14 by depositing its various layers 24 to 30 of different materials so as to form a stack of reflecting layers 24 to 30 of different materials on the substrate 14. Next, a bottom electrode 20, 32 is deposited on the acoustic mirror 12, and the piezolayer 19 of piezoelectric material is then deposited on the bottom electrode 20, 32 forming a so called piezolayer 19. Finally, a top electrode 21, 33 is deposited on the piezolayer 19. The combination of top and bottom electrodes 21, 20; 33, 32 and the piezolayer 19 is called the resonator section 11 of the device. The acoustic mirror 12 serves to reflect acoustic waves created by the resonator section 11 in response to a voltage applied to the piezolayer 19 across the electrodes 20, 21; 32, 33, thereby acoustically isolating the substrate 14 from the piezolayer 19.
What is fundamentally required from the reflecting layers 24 to 30 is to have a good reflection of acoustic energy created in the piezoelectric material, such that this energy does not leak out of the resonator, ultimately causing an undesired loss of energy.
The mechanical resonance frequency of a thin film bulk acoustic wave resonator filter 10 is determined by the time it takes the acoustic wave to make a trip from the top surface to the bottom, undergo a reflection, and return to the top. The thinner the device, the faster the wave returns. In a simplified view, the resonance, or sympathetic vibration, occurs at the frequency where a wave being input into the device constructively adds to the wave introduced in the previous cycle, but which has now returned to its original location. Thus the resonance frequency of the bulk acoustic wave resonator is set by the thickness and properties (i.e., speed, density) of the films deposited.
Bulk acoustic wave resonators are used as components for bandpass filters in e.g. the RF-section of a mobile phone. Such a filter may be a so-called ladder filter but can also be of the lattice type. A ladder filter, for example shown in FIG. 10, includes at least one so-called L-section, an L-section including a series resonator 22 and a shunt resonator 23 and thus consisting of an even number of resonators. In some applications, however, a filter consists of an odd number of resonators. For example, a 2½-stage filter could have either two series resonators 22 and three shunt resonators 23, or three series resonators 22 and two shunt resonators 23.
To create a bulk acoustic wave passband filter from such thin film bulk acoustic wave resonators, the shunt and series bulk acoustic wave resonators 22, 23 are manufactured so as to resonate at different frequencies (typically, but not necessarily with all the series bulk acoustic wave resonators 22 at one frequency, and all the shunt bulk acoustic wave resonators 23 at another). This is established by increasing the acoustic thickness of the shunt bulk acoustic wave resonators layer stack 23 as shown in FIG. 1 and corresponding description. Typically, the shunt bulk acoustic wave resonator's resonance frequency are reduced by adding a greater thickness Δde of material to its top electrode 33; for example by or depositing a shunt detuning layer 31 on top of the top electrode 33.
These filter applications of bulk acoustic wave resonators are typically operated in the fundamental, i.e. first order, resonance modes M1R, M2R as shown in FIG. 2 since these first order resonances M1R, M2R have the strongest coupling to the exciting electric field. Therefore, these first order resonance modes M1R, M2R achieve the broadest filter response. However, besides these first order modes M1R, M2R other acoustic-modes M1S, M2S can be observed. These higher modes M1R, M2R in the layer stack of the bulk acoustic wave resonator structure can be excited at higher frequencies. FIG. 2 illustrates this phenomenon and shows an impedance plot of two bulk acoustic wave resonators forming respectively a shunt and a series bulk acoustic wave resonator of a ladder filter. The strong peaks P1S, P2S at approximately 1800 MHz correspond to the fundamental resonance modes M1R, M2R of the two bulk acoustic wave resonators, 16, 17 shown in FIG. 1, whereas the weak peaks P1S, P2S at approximately 2800 MHz correspond to the above mentioned acoustic overmodes M1S, M2S. The dashed line 34 corresponds to the series bulk acoustic wave resonator 22, whereas the solid line corresponds to the shunt bulk acoustic wave resonator 23 shown in FIG. 1.
It is worth noticing that the over-mode MS shown in FIG. 3 does not correspond to the second or third or higher harmonics of the first order passband resonance mode MR. Actually the frequency of the acoustic overmode MS lies between the first order passband resonance mode MR and the second harmonic resonance mode not shown in FIG. 3. Just as the fundamental resonance modes M1R of the shunt and M2R of the series resonators create the filter passband according to the filter specification, the acoustic resonances M1S and M2S give rise to a “side passband” in the filter response which is shown in FIG. 3 where the corresponding transmission of the bulk acoustic wave passband filter is plotted as a function of the frequency.
Even though this side passband is typically quite narrow-banded and not very pronounced, it may fall into a frequency region in which strict stopband restrictions apply to maintain a proper predetermined filter characteristic in the respective application as shown for example in FIG. 3. Therefore, there is a need for a new bulk acoustic wave filter formed from a plurality of series bulk acoustic wave resonators and shunt bulk acoustic wave resonators, wherein each of said plurality of bulk acoustic wave resonators has a set of resonance frequencies. Thus it is an object of the present invention to provide a bulk acoustic wave filter having suppressed side passbands particularly within the stop band and to provide a method of manufacturing such a bulk acoustic wave filter.